WOT = less ah?

[pwbset]

Curious thing happening. I've got 6 Milwaukee packs in a 2p good, in theory, for 6ah, but in practice more like 5.2ah, which is what I budget for each day. Monday I rode home and the batts LVC tripped as I was pulling into the driveway - CA read ~4.8ah . Rained Tue/Wed so I drove ( ). This afternoon my drill pack LVCs were tripped while I was 2mi from home at 4.8ah again.. WTF? Did I just lose .4ah of capacity after only like 30 cycles? No way. I don't buy it.. these cells can handle a ton more than I'm throwing at them. All packs measured around the same voltage and all looked nominal before and after charge. The only thing I did different Monday and today was ride wide open throttle both times for most of the ride as I was testing some numbers. Now... I know I'm new at all this and this could be obvious, but I wasn't aware that riding balls to the wall could lower useable amp hours... use them up much more quickly sure, but lower actual capacity? 'Splain please.

PS - I've been messing with my Clyte controller shunt adding solder attempting to max amps slowly and so it's very likely I added enough by now to modify CA Advanced Rshunt and skew a bunch of CA readings, but there is no way that could effect actual drill pack built in LVC could it?

Signed,

 

[pwbset]

PS - I've been messing with my Clyte controller shunt adding solder attempting to max amps slowly and so it's very likely I added enough by now to modify CA Advanced Rshunt and skew a bunch of CA readings, but there is no way that could effect actual drill pack built in LVC could it?

Oh wait.. this is incredibly obvious now that I've written it down... because of the Rshunt being skewed now it's probable that I'm using the actual 5.2ah that I'm used to, but the CA is now only registering 4.8ah because it's not calibrated and since I've been riding flat out expecting to hit 5.2ah I'm just using juice way more than usual.

Ugh.

Now... about that pesky Rshunt calibration. Is there a way to calibrate a new CA Rshunt without a controlled power source/ammeter etc. Any way to hack calibrate it with a shoestring and some vaseline or something?? Or should I just "eyeball" it. I'd lower the CA Rshunt reading to display more amp/hr usage and get me back to 5.2 correct? Rshunt is currently 1.910mohm.

Signed,
Embarrassed and Confused

 

[dirty_d]

the faster you drain batteries the less capacity you get out of them, see http://en.wikipedia.org/wiki/Peukert%27s_law

 

[Link]

Peukert's only for lead...

 

[pwbset]

Okay... *&^% it... tomorrow is pay day and I'll go nuts not having calibrated gear so what exactly do I need to buy in order to calibrate a 72v 20A Clyte controller shunt to a Cycle Analyst and is there a HOW-TO (preferably visual) for calibrating this new Rshunt value? I know (at least I think I know) from various threads searched here that I need to apply a known current/load across the shunt (at what contact points?) and then measure the new resistance value in mohms and program that into the Advanced CA Rshunt setting and should be good to go yes within a percent or two yes? I have a feeling a lot of people (read: tinkering noobs who should know better by now ) would love to see a little step by step HOW-TO as they modify their controller current ceilings so I'm throwing in here to help create this visually if I just know the exact procedure etc. Thanks for any help!

 

[dirty_d]

Link, the effect happens for all batteries lead is just the worst, nicd and nimh are pretty good though i think even better than lithium with the p-effect. heres more details http://happytrailsmc.org/peukert.html

 

[Link]

Meh. True, but it's pretty much negligible for any newer chemistry.

 

[dogman dan]

The way we hammer our batteries on the ebikes, maybe not. I get what feels like voltage sag, but I think it's just the motor melting.

 

[docnjoj]

Nah! My Nimh sag almost as much as lead! Peukert lives in all batts, unfortunately! LifePo4 seem the least saggy!
otherDoc

 

[pwbset]

In the spirit of testing some stuff out this morning I rode as conservatively as I ever have on the throttle (see signature tracker link for geeky numbers if you're bored) and used a lot less amp hrs than usual. I pedaled about the same, but slowed my average speed down a lot.

I'm going to ride extra long on the way home (again lightly on the throttle) down to pack LVC and see what I get. If it's 4.8ah again I'll know for sure (90% sure anyway) that my Rshunt value needs adjusting (I've barely added any solder to the shunt so far)... if I get like 5.2ah I'll know that riding uber-hard WOT does lower my available amp hours by like .4ah, which would be a bummer and lead me to wonder how to re-condition LiMn for full capacity?

I'm averaging 4-8 volts of sag.. with 84v nominal available.. during my commute. i.e. After initial hot charge bleed off under load I'm seeing between 76-80 volts in 3s2p 6 pack of Milwaukees.

 

[ZapPat]

Guys - Voltage sag and peukert effect aren't the same thing. (WARNING concerning the http://happytrailsmc.org/peukert.html page posted above about peukert's effect: it says the peukert constant represents internal resistance - False! - check wiki's version instead: http://en.wikipedia.org/wiki/Peukert's_law).

Voltage drop is caused by the DC internal resistance of the cell, and varies a lot from one battery to the next, even using the same chemistry. From this effect you loose more voltage when you draw more current, and you loose more voltage also if your battery's DC internal resistance is higher too. Choose your pack wisely for this caracteristic...

Peukert's effect causes an apparent loss of usable Ah's the faster we draw the current from a battery. It is much lower for LiFePO4 than for lead, that is clear, but I've yet to read any technical tests to know how lithiums are affected by this effect. So we know that we do lose some capacity (Ah's) to this, but how much is yet to be shown.

So together, both these effects make you loose Watt-hours as you draw current faster, since Ah's times V's makes Wh's.

 

[pwbset]

So together, both these effects make you loose Watt-hours as you draw current faster, since Ah's times V's makes Wh's.

Thank you for the technical lesson ZapPat! My beginner brain appreciates the lessons! I didn't get a chance to test my "full range" under light throttle this afternoon as I popped two more spokes on the way home. Going to get my entire wheel re-laced and trued tomorrow. Hopefully that will solve that problem and I can get back to trying to figure out my apparent .4ah loss in capacity when riding full throttle. This is fascinating stuff! Wish I'd have studied it in college or something.

 

[John in CR]

Problem is everyone has left out the obvious. When you go faster the losses are higher due to wind resistance whose effect is not linear. In addition you'll have somewhat greater losses due to everything running hotter at WOT. ie hotter wires = greater resistance, in turn creating greater heat losses. The bottom line is that you DO use more electricity to travel the same distance at greater speed, so recalibration may be unnecessary.

John

 

[dogman dan]

Thanks, I'm slowly beginning to understand some of this stuff. The c rating thing has been particularly hard for me to grasp. I must be having real voltage sag then, since I hammer a Ping battery pretty hard on the way home, plus some more resistance as my motor runs hotter than it should. Still, when I talk about voltage sag, I'm still able to go 20 mph up a slight hill, so no complaining here compared to sla's.

 

[ZapPat]

Problem is everyone has left out the obvious. When you go faster the losses are higher due to wind resistance whose effect is not linear. In addition you'll have somewhat greater losses due to everything running hotter at WOT. ie hotter wires = greater resistance, in turn creating greater heat losses. The bottom line is that you DO use more electricity to travel the same distance at greater speed, so recalibration may be unnecessary.

John

More resistance in your wiring will not reduce your available Ah's, it will just give you slightly more voltage loss, thus also a loss of Wh's. The question here was not if we use more energy when going faster, but indeed if the battery's available Amp-hours go down as we drain them faster (Peukert style effect).
Making your shunt's resistance lower as you did will of course make it look like you are using less Ah's than you really are, but I couldn't help you with recalibration of your CA, pwbset, since I don't have one.

 

[John in CR]

Problem is everyone has left out the obvious. When you go faster the losses are higher due to wind resistance whose effect is not linear. In addition you'll have somewhat greater losses due to everything running hotter at WOT. ie hotter wires = greater resistance, in turn creating greater heat losses. The bottom line is that you DO use more electricity to travel the same distance at greater speed, so recalibration may be unnecessary.

John

More resistance in your wiring will not reduce your available Ah's, it will just give you slightly more voltage loss, thus also a loss of Wh's. The question here was not if we use more energy when going faster, but indeed if the battery's available Amp-hours go down as we drain them faster (Peukert style effect).
Making your shunt's resistance lower as you did will of course make it look like you are using less Ah's than you really are, but I couldn't help you with recalibration of your CA, pwbset, since I don't have one.

Sorry I read the original post wrong. The losses I was talking about would be after the CA. A quick question re Peukert's, is it just a heat loss at the battery or is the stored energy still in the battery but it can't release it because you sucked juice out too fast, since it can't just disappear?

John

 

[pwbset]

I couldn't help you with recalibration of your CA, pwbset, since I don't have one.

No sweat... I appreciate your patience and replies. I'm going to figure it out sooner or later. My wheel is in the shop as I type this being re-spoked and trued and I've filled my 20A controller shunt with solder so am expecting to see 30-40A. My thinking is that with the values really skewed I'll be able to see the effects more and re-adjust accordingly. I'll let you know how it goes after I extinguish the burning controller.

My final conclusion is that since I'm using brand new Milwaukee Emoli packs at 4C tops I can't possibly be losing .4ah amp hours due to Peukert so I must need to re-calibrate Rshunt anyway so might as well have some fun with it along the way at <deepboomingechoeffect>MAXIMUM AMPERAGE!</deepboomingechoeffect>

 

[Freddyflatfoot]

Just been reading this thread. I use a Ping battery as well, 36v/10AHr, on a Bafang motor. Wind could do it, I can use anywhere between 1.6 and 2.2 AHr's riding to work, depending on the wind, traffic conditions, and how hard I am hammering the system, by not pedaling, or running full throttle.
Another issue I have discovered, is that these batteries WILL lose some capacity in cooler weather. I have been noticing this
(it is winter here), and noticed that my battery is sagging way more than when I first got it, (down to as low as 32.3 v).
I have e-mailed Ping, and he has confirmed that these batteries will lose capacity and performance in cooler weather, but he has assured me that things will improve when the weather warms up.
Unfortunately, I'm probably going to have to wait a while to prove if this is the case!